Rear end build advice
 

Hi Guys,

Just ordered my BDR #474 From Jay. I'm looking for opinions about rear end set-ups.

So far here's what I have. Going with a 351-AFR heads, lopy cam... that will be just shy of 500HP. 17 inch wheels with the F1 tires (315's rear). Tremac 500-5 speed.

I know it's going to be a BMW E36 rear end-but I am having trouble with the end decision.

I plan on using for street mostly. Just having fun. Maybe some track in the future when I get used to it. Not alot of Drag. Just a driver-with a hell of a pickup.

My question is about an open axle-vs LSD-vs-POSI. And what ratio. I have heard that the 3.15 is a dog, and I have heard the opposite from some guys that have it (and cruise at low RPM on the freeway) Is a 3.46 a better bet? I think a 3.91 would be a little too low.

Also they offer different lock settings-I am leaning towards a 3.46 with a 40% lock.

Any advice is totally appreciated. I also totally appreciated the advice from Jay, Michael, and Len. Just wanna put it out there and see what sticks.

I am ok with a range of $500-$1500 to build this rear end right. Rather do it once....


-Sean

I'm voting for 3.73 ;) even though I have a 3.15 LSD on the shelf.

rear
 

I have a 3:73 with a 4 speed top loader, 3000 rpm @ 60 MPH if you go with a 5 speed that is probably the gear to have.

I know you will be a lot happier with something like a 3.73 and probably nothing higher than 3.46/3.50. Your first gear may be a little low but if you can dictate that you want a .82 5th gear reather than a .63 you will be even happier yet.



Clois

TKO-500 Transmission ratios:

1st 3.27
2nd 1.98
3rd 1.34
4th 1.00
5th 0.68

Thanks for the advice. I'll talk to Jay about the .82 final drive ratio. They are putting the drivetrain together currently.

My first engine was a 351 AFR heads stage II trick flow cam and it dyno'ed at 326 RWHP. MY rear was a 3:10 open differential. Transmission was a T-5 (turns out from a 6 cylinder mustang).
My best track time was 12.2 et at 112. That engine went south doing an open track event (check my pics). My new engine a dart block 408 CI, same AFR heads stage III cam, victor Jr. with a 950 cfm Holley HP. Changed my rear to 3:73 with three clutch packs added. And a TKO 600 with a .68 5th gear. Dyno'ed at 428 RWHP.
Trip to the drags. Best of 12.2 at 120mph. Now this could get better but it is such a hassle getting through tech inspection at an NHRA track that I don't bother anymore. I use the car mostly for open track events where it eats German and Bow-tie cars (except for new Z06's). I am thinking about changing the 5th gear to the .82 because on a long straight I wind out 4th and a shift into 5th sounds like the engine goes to sleep. The best thing would be to have a T-56 6 speed but my BDR is older and would need trans mission tunnel mods, which I don't feel like doing.

Bottom line, for around town and some spirited highway driving your stock 3:15 rear will be fine. I would seriously question the 500 hp number, sounds like you should have something like 300-350rwhp.

Should be more than that.... but 500hp is at the crank. As most engines are sold ;)

Casbburn, you are a knowledgeable guy, how can you say that a 351 with AFR heads and a lopy cam will produce 500 plus hp even on an engine stand with the builders headers and no accessories? Maybe with a 150 hp shot of NOS.

Engine builders may sell them with crank ratings for HP and torque, but we all know (or should) that everything changes when we put on the cobra headers and mufflers and normal engine accessories. Then there is the power train loss, 18 to 20 %.
I stand by my 300to 350 number mounted in the BDR. Would love to have your customer share his dyno readings once everything is installed.

Then again I met a Cobra guy who was told a 500 plus hp number, but always had an excuse for not putting it on a dyno to find the real number.
Down deep inside he would rather believe (and boast) the 500hp number than learn the truth.

Jaybar,

I was interested in getting as much advice as possible. I am sure there will be a redux at the rear wheels. And when I get my car, I'll be happy to dyno it and share the results if people are interested.

That being said, let's not make this about anything other than having fun and building cars that help us get there.

My figure wasn't a boast by any means just a stated figure from a build sheet so folks knew what it was I was shooting for. I was looking for advice about gear ratios from people that have had a set up just like mine, as it appeares you have. I appreciate the input.

whiplash congrts on new BDR
 

I have BDR # 435. Jay & Brian @ Vintage built my car.You are in good hands as these guys are the best.My car Has a Performance Engineering 351w.It made 435 hp on the dyno. I don't know what the rear wheel HP is but I can tell you it is CRAZY FAST!!!!!. I have the 3:46 rear with a TKO 600 trans.5th gear is a little low but it is also nice to cruise on the Parkway( 70 MPH @ 2200-2300 RPM) So welcome to the BDR and Vintage family and I hope you have lots of safe miles. The fun, cool stuff, driving and ear to ear smile just come with having a Cobra.

I cannot say that, nor did I. The target is 500hp (I was simply cuing Sean's number), in reality it will likely be 485HP to make the thing run ok on poor CA 91 Octane and do it safely. If someone sold you a Cobra and told you it was 500HP and you expected it at the wheels, then you've got a bone to pick with them (or yourself depending on where the misunderstanding lies). :rolleyes: you may also want to consider the difference between "shy of" and "plus" ;)

You expect an engine builder to promise a RWHP number? **)

You really want to get into the dyno debate??? There's more mystery to that my friend. Dyno racing is another topic saved for face to face discussion.

Sean,
With the 3.73s and the .68 overdrive of the TKO500... you will be turning 2200 RPM at 65 MPH. This is calculated with the 17" Eagle rear tire that is about 25.25" in diameter.

If you drop down to a 3.46 with this setup you are only dropping to 2050 RPM. Better to have the extra off the line...

Me personally I would go with 3.73, I had that in my cobra. It was extremely quick and was plent fast for me. On the chassis dyno it would do 165 mph at 6000 in fifth gear with a TKO with .68 overdrive. Now this motor would turn over 7500 rpm but I used a 7500 chip in the rev-limiter. I fairly sure it would pull that in high gear just didn't need it don't like high speeds. I then put in a TKO 600 with the .82 over and loved it. Would definately reccomend that set-up. I had Bimmerworld do my diff, it was $1600 with a 50% lockup worked real nice on the street. By the way my motor made 612 horse 615 torque on engine dyno, made 460 horse 470 torque to the rear wheels. I also ran race fuel 110 octane in her all the time, but didn't need it it was really a pump gas motor. With the aluminum block my BDR tipped the scales at 2200 pounds. Go with the 3.73, I prmise you will love it.

DON

Thanks all for your well thought out answers to my questions. If ya see me in Reno at WSCB this May-I owe ya a cold frosty one. I'll be the guy with the stupid grin on his face. :)

Jay-There is a 3.73 LSD on the way from Metrix Motorsports to CT. Looking forward to it!

-Sean

Ahh Whiplash, you have hit on 2 of my favorite subjects, but I will try and keep it brief.

Hp/tq is relatively difficult to predict, but engine builders that repeat a particular build can get very close to what you will actually get. Conversely, you cannot predict hp/tq by knowing a few of someone’s engine specs. I have seen rwhp for many cobras in our club. 351's are typically around 350-375 rwhp, 393's are typically 390-430 rwhp (I built my 393 and got lucky with just over 470 rwhp) there are only a couple 408-427 small blocks in our club and I'm not sure what their numbers are. I do know of a fuel injected 427 with a 9lb blower that made 650 at the crank, but the fuel delivery is questionable so that number is somewhat unreliable. I have been told by an owner of a dyno that has seen the crank numbers and then the rwhp numbers. He states on average you will lose 60-70 with a manual trans and 90-110 with an automatic.

Diff ratios are absolutely dependent on how you will drive the car and what you expect from it. BDR almost always ship their cars with a 3.10 open diff and 25% lockup. That is actually a wise choice on their part and is debatable better for road racing which is the spirit of their cars. An open diff is much more forgiving to mistakes and that is what most guys will be doing when they first get behind the wheel of these cars. My car is much more harder to handle now with a 3.64 ls after breaking the output shaft on the original open 3.46 (like I said "almost always"). It is important to note that it happened on a quick wheel hop rather than a total torque strain and at the time I had 3200 miles of very spirited driving and several trips to the drag strip. It is amazing what these little bimmer diff will take, but they do have limits. It happened in front of the Hampton after the London show, but thankfully after the main street event....I would have hated to miss that. Anyway, BDR was very good about sending me a replacement diff that was limited slip and the gear ratio I wanted. The biggest reason I am glad to have a limited slip diff is the more positive spread of power to both wheels, which make it squirrelly. I still have the 15" BFG's which further bears out their weakness for traction. I must switch to 17" to get a decent tire selection or drive around with drag radials, which is the only alternative for 15" tires that are 295s. With the added traction will come the added strain on the e36 irs. If you can get the m3, do it! You will never regret it.

Here are what I consider to be the hot setups:

400+ rwhp, t56 6speed, 3.73 ls diff with 50% lock on m3 irs, 17" rims

400+ rwhp, tko with .82, 3.64 or 3.73 ls diff with 50% lock on m3 irs, 17" rims

I think I said I would keep it brief and I really did try. Sorry if it all seems a little cryptic. %/

One more time,

OK, I just want to leave two big black stripes when I drop the clutch. I've got the standard (I think) 3.10 open with a Tremec 3550 (.68 in 5th). Should I be looking at a 3.73 or the 3.46? I have a 351W with about 400hp at the crank (not sure of rwhp).

My concern is running out of first gear too fast at it is a 3.27 ratio. Wouldn't the 3.46 allow me to shift later out of first?

Also, where is the best place to get a limited slip unit for a Backdraft. Bimmerworld has been mentioned. Any others?

SkipB

Hi Skip,

I'll leave the gear ratio discussions to the guys that know and or build them. I'll give ya my personal experience in about 5 weeks. (right Jay? :) )

I have had alot of people say the 5 speed with the final drive of .82 is nice (especially if you are going to track it).

I got my rear here:

http://estore.websitepros.com/139711...103cb27e798804

There were also some decent deals on e-bay at the time on the 3.46 LSD. Just type in e36 LSD and see what comes up.

this next site appears a little older-but might be worth a look/call

http://www.diffsonline.com/product/used.htm

Hope that helps in your search.

-Sean

Sean,
They loaded the truck today. I have to speak with Eric tomorrow (another order) so I will get a feel for engine day. Have to wait longer for the AFRs vs. the Edelbrocks.

Yep-no worries Jay. I had some specific special requests as far as the engine goes-so if it takes a little longer I totally understand. The extended family wants to see more pics though-;) Everyone is excited